Arm64 Debian build fixes

The goal of this MR is to have Purr build correctly on the OBS for arm64 Debian systems (Buster a.k.a. Debian 10 for now), so that it can be run on Chromebooks with a true 64 bit Linux subsystem, but it's not quite there yet. The problem is that we don't have any proper arm64/aarch64 packages for nw.js yet, so we're using Leonard Laszlo's armv7 (32 bit) packages instead.

What works: The package builds correctly and all regression tests pass. So everything but the gui works.

What doesn't work yet: The final packaging fails during the dpkg-shlibdeps phase, when dpkg detects that it can't find the 32 bit system libraries needed to run nw.js.

I guess that the package would work fine if I knew what 32 bit library dependencies are needed to make the armv7 version of nw.js work on an arm64 Debian system.

Jonathan, can you help with this? I know that you're running Purr on your Chromebook, is that a 32 or 64 bit Debian that you're running there? Maybe what I'm trying to do just isn't possible, so running Purr only works on 32 bit arm Debian systems for now?